1. Field of the Invention
The present invention relates to an industrial polyester multifilamentary yarn of high modulus and low shrinkage, as a reinforcement for tires, and a dipped cord formed therefrom. More particularly, the present invention relates to a polyester multifilamentary yarn which retains superior dimensional stability and fatigue resistance even at high temperatures and a dipped cord formed therefrom. Also, the present invention is concerned with a method for producing such a polyester multlfilamentary yarn and a dipped cord.
2. Description of the Prior Art
One of the typical functional uses which fibers have is to reinforce rubber composites, such as tires. Examples of the fibers useful as such a reinforcement include nylon, polyester, rayon, etc. Of them, polyester fibers contain benzene rings in their molecular structure, exhibiting a rigid property. Accordingly, tire cords produced from polyester yarns shows high elastic modulus and few flat spots with superiority in fatigue resistance, creep resistance and endurance. By virtue of these high physical properties, polyester is extensively used as a reinforcement for rubber composites, especially tires.
In spite of these advantages, conventional polyester tire cords suffer from a significant disadvantage of reducing the side wall indentation of monoply radial tires. Also, industrial polyester yarns are required to improve in dimensional stability in order to replace the rayon fibers which have been applied for radial tires. In this regard, recent research has been directed to the development of polyester fibers which have high strength and elastic modulus in the same level as that of rayon fibers.
Techniques for increasing thermal stability in polyester fibers are found in, for example, U.S. Pat. Nos. 4,101,525 and 4,195,025 (both to Davis et al.) which disclose a polyester tire cord produced by drawing highly oriented undrawn yarn in a high-speed spinning process under a steaming condition to give highly oriented drawn yarn, especially multi-drawn yarn containing at least 85 mol % of polyethylene terephthalate, which ranges, in denier per filament, from 1 to 20 and, in work loss at 150xc2x0 C., from 0.004 to 0.02 lbxc2x7in, and dipping the multi-drawn yarn in a rubber solution.
Another prior art relating to a tire cord can be acquired from Japanese Pat. Laid-Open No. Sho. 61-12952 which discloses a process for producing a tire cord, comprising the steps of spinning a polyester having an intrinsic viscosity of 1.0, a diethylene glycol content of 1.0 mol %, a carboxyl group content of 10 eq/106 g at a spinning speed of 2,000xcx9c2,500 m/min to obtain undrawn yarn, drawing the undrawn yarn at about 160xc2x0 C., thermally treating the yarn at 210xcx9c240xc2x0 C., and dipping the yarn in an ordinary rubber solution. In this process, the temperature just below a spinning nozzle ranges from 100 to 450xc2x0 C. The tire cord thus produced is, however, poor in physical properties. For instance, the tire cord ranges, in absorption peak temperature in amorphous portions, from 148 to 154xc2x0 C. and, in dry shrinkage, from 3.3 to 5% with a tenacity of at least 7.0 g/d.
Focusing on high tenacity and low shrinkage, as introduced above, the research which was made on the development of the filamentary yarn for tire cords provided methods in which undrawn yarn with a high quantity of orientation and crystallinity is produced through spinning at a high stress and endowed with high tenacity and low shrinkage properties through drawing at a high draw ratio.
The yarns produced by the high-speed spinning or drawing according to the prior arts are improved in fatigue resistance, but problematic in that the molecular chain lengths in amorphous portions are non-uniform and extend. As a result, relaxed molecular chains coexist, giving rise to a great loss in tenacity. Thus, the yarns suffer from significant disadvantages of being poor in drawability owing to a large difference in physical properties between inner and outer layers of the yarn and of exhibiting a great variation in physical properties owing to defects in their micro structure. Moreover, the yarns produced from a highly viscous polymer with an intrinsic viscosity of 1.0 or more show a limit of low shrinkage. Yarns which are drawn with a high orientation in advance of undergoing a tire cord conversion process have a definite two-phase structure of crystalline and amorphous portions. Where the highly oriented yarns are subjected to a thermal treatment by dipping in a rubber solution, deterioration is brought about in the crystalline portion with aggravation in the non-uniformity of the molecular chain, leading to a lowering of strength. As for polyester multi-filamentary yarn, it is highly apt to be damaged because it undergoes a series of after treatment processes. For example, at least two strands of the drawn yarn primarily obtained are subjecting to first and second twisting and formed into a fabric, after which the fabric is dipped in a rubber solution and incorporated into a rubber matrix of a tire, and during these processes, the yarn may be changed in physical properties and undergoes breaking of molecular chains.
Knowledge of the fact that, in order to use drawn polyester filamentary yarn in tire cords, it is important to allow the drawn yarn to have a uniform structure of molecular chains and the balance of high elastic modulus and low shrinkage than to provide the drawn yarn with high tenacity and low shrinkage because the drawn yarn experiences serious alteration in physical properties and molecular structure, leads to the present invention.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide polyester multi-filamentary yarn for tire cords, which retains excellent thermal stability and fatigue resistance even after thermal aging.
It is another object of the present invention to provide tire cords formed from such polyester multi-filamentary yarn.
It is a further object of the present invention to provide a method for making such polyester multi-filamentary yarn for tire cords.
In order to approach the above objects, first, when making drawn yarn, factors causative of non-uniformity in the molecular chain structure of the yarn must be excluded to as much extent as possible and the balance of high elastic modulus and low shrinkage is provided for the yarn. Next, the drawn yarn is allowed to undergo a uniform structural change in the course of the dipping process and even under the strict conditions of tire manufacturing processes, so that the resulting dipped tire cords are deformed as little as possible under a high temperature condition of the tires, i.e. tire rotation while the car is driving. Consequently, the tires are superb in endurability.
In other words, the factors which cause non-uniformity in the molecular chain of the yarn are minimized during the producing processes of the drawn yarn while the parameters which are involved in the structural change of the molecular chains of the drawn yarn and dipped tire cords are controlled in the dipping process and tire-manufacturing processes.
There are many factors which are causative of non-uniformity in the molecular chains of polyester yarn. For instance, in the course from the melting of a polyester resin to the step just before the extrusion of the molten polyester from nozzles, the intrinsic viscosity and melting temperature of the polyester resin have an influence on the molecular weight distribution of the molten polymer, together with the retention time which it takes for the molten polymer to flow to the nozzles. Upon extrusion of the molten polymer from the nozzle, the number and the diameter of the nozzles play an important role in determining the uniformity of the resulting yarn. In the processes after the extrusion, such as a quenching process and a winding process, quenching temperatures and winding speeds cause a structural change in both of the inner and outer layers of the yarn extruded from the nozzles (hereinafter referred to as xe2x80x9cextruded yarnxe2x80x9d), thereby bringing about a significant effect in the molecular chains of the inner and outer layers. During the drawing of the extruded yarn by taking up, the causative factors included the orientation and breakage of the molecular chain. Upon thermal treatment, the relaxation extent of the molecular chain is taken into account. Hence, the formation of a uniform structure in the molecular chain is affected by a variety of factors which are distributed in various process steps from polymer melting through melt spinning, quenching quenching temperature), and drawing to thermal treatment. Since the factors are interconnective to each other, an appropriate combination of the factors is necessary to produce the drawn yarn which has a uniform structure of the molecular chain.
Fundamentally, in order to attain a uniform structure in the molecular chain, the processing conditions at the process steps which are important for the uniformity of the molecular chain are set in such a manner that the occurrence of the non-uniformity is minimized. For instance, it is preferable to minimize the retention time in the melting and filtering steps of a polymer. The non-uniformity due to a sudden change in a quenching step after spinning can be significantly reduced by converting the sudden change into a gradual one. Where the non-uniformity of the molecular chain is due to drawing, it can be solved by conducting the drawing process at a low draw ratio. In addition, a thermal treatment stabilizes the molecular chain.
Accordingly, the present invention can be attained by satisfying the conditions of the problematic process steps simultaneously.
In accordance with an aspect of the present invention, there is provided a process for producing polyester filamentary yarn from a polyester resin which comprises at least 90 mol % of polyethylene terephthalate with an intrinsic viscosity of 0.7xcx9c1.2, comprising the steps of: melting the polyester resin at a temperature of 290xc2x0 C. or below; filtering the molten resin for a filtering retention time of 10 min or below; extrusion-spinning the filtered, molten resin through a nozzle which has 250xcx9c500 holes, each ranging, in diameter from 0.5 to 1.2 mm with a length/diameter ratio from 2 to 5; primarily quenching the extruded yarn at a temperature of 100xcx9c195xc2x0 C. in a zone of 50 mm or more distance directly below the nozzle; secondarily quenching the yarn with quench air at its glass transition temperature (Tg) or below; taking off the yarn at a spinning stress of 0.3 g/d or greater; and drawing the taken-off yarn at a total draw ratio of 1.3 or greater and thermally treating the yarn at a temperature of 150xcx9c230xc2x0 C.
The term xe2x80x9cfiltering retention timexe2x80x9d as used herein means the time it takes for the molten resin to travel from the screw end of the extruder to the holes of the nozzle.
In accordance with another aspect of the present invention, there is provided polyester filamentary yarn for tire cords, which comprises at least 90 mol % of polyethylene terephthalate and has an intrinsic viscosity of 0.70xcx9c1.2 and a tenacity of 5.5xcx9c8.5 g/d with an intermediate elongation difference (E1xe2x88x92E0) between intermediate elongations E0 and E1 amounting to 6% or greater. The intermediate elongation E0 is the elongation under a load of 4.5 g/d while the intermediate elongation E1 is the elongation under a load of 4.5 g/d after conducting a thermal treatment at 177xc2x0 C. for 10 min under a load of 0.01 g/d. The yarn preferably has an amorphous orientation function of 0.65 or greater and a terminal modulus of 15 g/d or below.
In accordance with a further aspect of the present invention, there is provided a polyester dipped cord which is produced by twisting the polyester filament yarn in at least two strands, forming the strands into a fabric, and treating the fabric with blocked isocyanate and resorcinol formaldehyde latex (RFL), wherein the cord satisfies the following characteristics:
i) a tenacity of 5.0 g/d or greater,
ii) a dimensional stability index (E4.5+SR) of less than 7.0%,
iii) a breaking elongation of 9% or greater,
iv) an intermediate elongation difference (E1xe2x88x92E0) of 3% or less.